I have had a box of negatives and prints scanned, and I am having a box of old slides scanned. Since some of the slides predate me, there is not much metadata to capture, other than the fact that it is a scan from a slide (and if I'm lucky, the location and an estimated date).
However, for my negatives, I have a lot more information available. I know I can create custom xmp tags to store the data, but I realised I would not be the first person who wanted to record the film type, exposure number and other details. Based on my limited scripting experience, I started looking for existing data structures. Recording this data in a custom structure, means there is no hope of any software ever reading it.
The FlashPix ImageInfo tags appear to have all the fields one would need, and already have exiftool support. However, these tags are write-only. Is there a reason why these are not writable?
Is there an easier, better solution?
From reading the details on the FlashPix tags, it doesn't seem like Phil has a high opinion of the format. Plus, there probably hasn't been any interest in writing to that format.
It's also quite unlikely that very many programs will read these tags, leading to the same problem as creating your own custom format.
Personally, I am a fan of keeping data that doesn't have an already preexisting tag in IPTC:Keywords/XMP:Subject or XMP:HierarchicalSubject. I feel HierarchicalSubject is especially good for this because it can be categorized as you like. Modern programs like Lightroom will use these tags, making it easier to find and sort based upon that data if you like.